Solar cells are used as photovoltaic elements for converting light into electrical energy. To that end, various doped regions are provided in a semiconductor substrate such as, for example, a silicon wafer. The doped regions can differ on the one hand as regards their doping polarity, that is to say they can be n-type doped or p-type doped; on the other hand, the doped regions can also differ as regards the doping concentration, that is to say as regards the density of dopants within the volume of the doped region. On account of different types or densities of charge carriers within the various doped regions, a potential difference is produced at boundaries between adjacent doped regions. By means of such a potential difference, charge carrier pairs which have been generated in the vicinity of those boundaries by absorption of light can be spatially separated.
Many solar cell concepts have already been developed in which a plurality of different doped regions are formed adjacent to a surface of a semiconductor substrate, for example by the purposive diffusion of dopants. Doped regions formed in that manner generally have a doping concentration which is substantially higher than a base doping concentration originally prevailing in the semiconductor substrate. For example, the doped regions have a doping concentration of typically more than 1*1018 cm−3.
For example, a simple conventional solar cell typically has on its front side which is to be oriented towards the sun an areal emitter region which has an opposite doping polarity to the base doping of the semiconductor substrate and a substantially higher doping concentration. On their back side, such solar cells generally have a doped region, referred to as BSF (back surface field), which has the same doping polarity as the base doping of the semiconductor substrate but possesses a substantially higher doping concentration.
In order to be able to feed the spatially separated charge carrier pairs generated in the semiconductor substrate under incident light to an external electric circuit, the semiconductor substrate is contacted via electrical contacts, base contacts contacting the BSF and emitter contacts contacting the emitter region in the example mentioned above. It has been found to be advantageous to design the emitter region to be thicker and/or to have a higher doping concentration in partial regions adjacent to the emitter contacts than in intermediate partial regions. This is referred to as a selective emitter, wherein the partial regions adjacent to the emitter contacts can be optimised in respect of an electrical contact resistance, whereas the intermediate partial regions can be optimised with regard to low recombination losses and accordingly as efficient a quantum yield as possible.
In an alternative solar cell concept, both types of contact are formed on the back side of the semiconductor substrate facing away from the sunlight. It is hereby possible, for example, for emitter doped regions and base doped regions to be formed alternately next to one another, for example in an interlaced arrangement, and to be contacted in each case by emitter and base contacts which are likewise arranged in an interlaced manner.
The various doped regions can be produced by means of different processing steps. For example, dopants can be formed by diffusion from a gas source that is to be considered inexhaustible, by diffusion from a solid dopant source applied temporarily, or by ion implantation. Different doping profiles are thereby obtained within the doped regions according to the type of processing, that is to say the doped regions can differ in particular as regards a surface doping concentration, as regards a profile depth, as regards a sheet resistance and, where appropriate, as regards a thickness of a cover layer such as, for example, a dopant-containing glass. The doping profiles resulting from the processing may frequently not be optimally configured for their intended use in the solar cell. It has therefore been found to be advantageous in many cases purposively to etch-back doped regions after their production. This is also referred to as “stain-etch” or “etch-back”.
Inter alia because of their frequently very different etchabilities by means of etching media, different doped regions on a semiconductor substrate have hitherto been etched-back in separate processing steps in solar cell production. Regions of the surface of the semiconductor substrate that are not to be etched are typically protected temporarily by etch masks, for example, and then the unprotected regions are etched-back by means of an etching operation which has specifically been optimised for the doped region that is to be etched-back and for the doping profile that is to be achieved.
In conventional solar cell production, this results in a considerable outlay in terms of work and material in order to etch-back the various doped regions.